Maintainers.txt: Update maintainers/reviewers for CI and Test

Maintainers.txt: Update maintainers/reviewers for CI and Test

Remove Bret Barkelew from:
- .azurepipelines
- .mergify
- .pytool
- UnitTestFrameworkPkg

Add Michael Kubacki to:
- .azurepipelines
- .pytool

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Michael Kubacki <mikuback@linux.microsoft.com>
Signed-off-by: Bret Barkelew <bret.barkelew@microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>